Sigil Stone

When you remove one of these from an Oblivion gate to close it, you'll find that it has certain effects associated with it. These will either be a constant effect for armor or an on-strike effect for a weapon. The constant effect enchantments are much more useful. You can use Sigil Stones to enchant any piece of armor or weapon that doesn't currently have an enchantment without even having to go to the Arcane University. The effects of the Sigil Stone increase as you level up, but they are still random. If you save your game right before pulling it out, you can reload the save and try again if you find you got a stone with a useless enchantment.